Data storage media and mechanisms (e.g., disk drives) generally exhibit increasingly higher capacity devices to be competitive in the marketplace. As demands for data storage capacity increase, data bits (e.g., magnetically recorded bits) are generally packed more densely into the same recording medium dimensions to achieve higher capacity in the same form factor drive.
One of the fundamental parameters in the design of magnetic recording systems is the physical space between the recording head and the data recording medium (e.g., a rotating hard disk). The transducer in the head may perform both playback (read) and record (write) functions, and thus, may be known as a read/write head. The quality of recorded data bits (e.g., magnetic transitions) and the playback signal strongly depend on the clearance (or spacing) between the slider on the read/write head and the disk. This spacing is also known as flying height or the “fly height.”
Various factors affect the read/write head-hard disk clearance during read and write operations, and can cause modulation of (or variances within) this spacing. In a magnetic data storage system, the speed of the disk rotation, the slider air bearing design, smoothness or roughness of the recording medium surface, operating altitude and temperature are some of the key factors.
FIG. 1 shows a conventional magnetic data recording and playback system 10, including read/write head 20 having write transducer (or coil) 30 and read transducer 40 electrically attached thereto. Electrical current passing through write coil 30 during a write operation generally heats the coil 30 and causes it to expand, reducing the spacing between write coil 30 and medium (or disk) 50. Protrusion of the recording element (write head) during the write process due to Joule heating and eddy-current losses may significantly reduce the flying height of sliders in hard disk drives. Such thermal expansion of the write coil 30 can also affect the position of the read transducer 40 relative to the disk 50. In some cases, the thermal expansion and contraction of the write coil 30 can be a primary factor in the variation of the fly height of the read/write head 20 and/or write coil 30.
After the recording system (e.g., through firmware in a disk drive) commands the drive servo to position the write head 20 on-track (e.g., at the beginning of a write operation) and a data read/write controller (such as a hard disk controller, or HDC) asserts a write enable signal (e.g., write gate or WG), a circuit such as a preamp sends current through the write head coil 30. The current passing through the coil 30 generates thermal power or energy, which causes the pole tip 35 to protrude towards the disk. The pole tip protrusion (PTP) generally reduces the magnetic spacing between the head and the disk 50.
On the other hand, when the write enable signal is deasserted, the current flow into or through) the write coil 30 is reduced or stopped, and the thermal energy stored in the pole tip 35 begins to dissipate into the air and the surrounding coil insulation material. The decay in thermal power (e.g., the rate of decrease in stored thermal energy in the coil 30) from the write operation causes the pole 35 tip to retract to its original position (e.g., to the original spacing). The rate of thermal power increase (during writing) and decay (when not writing) are different. This difference can create an undesirable modulation in spacing between the write head 20 or coil 30 and the data recording medium 50, which can impact the data integrity and bit error rate (BER) of the drive.
One method to adjust the fly height spacing between a magnetic recording medium and the read/write head involves thermally heating the transducer region (in the write head) with a heater element. However, after a write operation ends, current no longer passes through the coil 30, and the write coil 30 may begin to cool and contract. Importantly, the rate of thermal expansion during a write operation may not be the same as the rate of thermal contraction when current is not passing through the write coil 30. The different rates of thermal expansion and contraction give rise to a type of hysteresis in the fly height variation, generally as a function of actively writing data versus not writing data. Thus, it is believed that a simple heater element (that may not take the different rates of thermal expansion and contraction into account) may not be capable of maintaining the write coil 30 at a relatively constant distance above the data recording medium 50.
Furthermore, the start of a write cycle (known as “cold write”) requires relatively high write current, when the coil temperature is relatively low, in order to overcome an increase in media coercivity. The write transducer-recording medium spacing modulation can be further complicated by the use of a heater element, as mentioned above.
A need therefore exists to reduce the variation and/or temperature-based modulation in write transducer fly heights, to keep up with ever-increasing demands for increased data densities and operational write speeds, as well as improved data integrity and reduced bit error rates, in high-speed data recording systems.